Adverse yaw is when the drag from the down traveling aileron causes the plane to yaw away from the bank so the plane looks like it's crabbing in the turn. It shows up mostly with sailplanes and their long, high aspect ratio wings. It's really not a big deal with power models. And even at it's worst it won't cause a spin or snap roll.

How did the weight of this Wagon compare with the other one? I'm wondering if you have a higher wing loading than the first one?

I'm still kind of suspecting that the wing may have had a bad twist in it. If not then I guess we're back to the nose heavy thing.

Um, what sort of shape does the leading edge of this wing have compared to the other one that flew well? If the leading edge is shaped sharper on this one then we may have another clue.